In recent years, the demand of a high strength steel sheet has been increasing more and more with the aim of such as lowering the fuel cost by reducing the vehicular body weight of an automobile and the like and securing safety in collision. In accordance with that, the demand on the tensile strength of the steel sheet also has been increasing, and a high strength steel sheet of 780 MPa class or above is required instead of a low strength steel sheet of 590 MPa class. However, when the tensile strength becomes 780 MPa class or above, deterioration of formability is inevitable, and in particular, deterioration of bending workability becomes a problem. Bending work is roughly divided, according to bending direction, to rolling direction bending [bending in which the bending axis is the direction perpendicular to the rolling direction (L direction)] and sheet-width direction bending [bending in which the bending axis is parallel (C direction) to the rolling direction (C direction)]. In a low strength steel sheet of 590 MPa class, both bending work can be performed comparatively easily, however, as the tensile strength becomes higher, bending work in C direction becomes difficult, and bending work in L direction which is said to be easy to perform bending work compared to that in C direction is liable to become difficult as well.
As a high strength steel sheet excellent in bending workability, a dual-phase steel sheet in which the ferrite phase and the low-temperature transformation phase such as martensite and bainite co-exist is used. The dual-phase steel sheet is one enabling improvement of both strength and workability simultaneously by dispersing the hard low-temperature transformation phase in soft ferritic matrix, and the methods described in the Patent Document 1 to Patent Document 5, for example, have been proposed.
The Patent Document 1 was proposed by the applicant of the present application and describes a method for improving bending workability by controlling the number of oxide-based inclusions present in a fracture. The Patent Document 2 describes a method for preventing a crack during bending work by formation of bainite including carbide and/or martensite-including carbide. The Patent Document 3 describes that elongation, stretch flange formability, and bending workability when bent in the rolling direction (L direction) are improved by optimization of the ferritic grain size and the fraction and hardness of a phase formed by low-temperature transformation. The Patent Document 4 describes a method for securing bending workability by lowering the hardness of a surface layer than that of the inner part and suppressing variation of Vickers hardness of the inner part in a high strength steel sheet mainly of bainite or martensite. The Patent Document 5 discloses a high tensile strength steel sheet excellent in bending workability in any direction of rolling direction bending, width direction bending, and 45 degree direction bending (bending with the bending axis direction inclined by 45 degrees against the rolling direction) realized by heating steel with a specific chemical composition and appropriately controlling the hot rolling condition (particularly hot finishing rolling temperature, cooling rate thereafter, and winding temperature) and the annealing condition (annealing temperature and cooling rate thereafter).
On the other hand, in order to make the above described high strength steel sheet thin to adapt automobile components and the like, it is necessary to be excellent in fatigue strength. The reason is that the stress during traveling of an automobile increases by thinning, therefore the risk of fatigue failure increases if the fatigue strength is low. However, the fatigue strength is not considered in the above Patent Documents.
[Patent document 1] Japanese Unexamined Patent Application Publication No. 2002-363694
[Patent document 2] Japanese Unexamined Patent Application Publication No. 2004-68050
[Patent document 3] Japanese Unexamined Patent Application Publication No. 2005-171321
[Patent document 4] Japanese Unexamined Patent Application Publication No. 2006-70328
[Patent document 5] Japanese Unexamined Patent Application Publication No. 2001-335890